In dealing with conveyer belts, especially belts in gravimetric feeder systems having weighing functions, it is paramount that the belt be extremely flexible so that in the weighing zone the proper impact of weight can be fully transmitted through the belt. The most flexible belt, of course, would be one made entirely of rubber or other elastomer. However, due to stresses on the belt, it has been necessary to reinforce the belt with a ply of fabric, preferably tough textile fabric such as polyester or rayon.
Since the introduction of gravimetric feeder systems in the 1950's, manufacturers have insisted that in order to maintain .+-.0.5 percent accuracy (i.e., precision or repeatability) the operator of the feeder should only use an endless belt, i.e., a belt having no mechanical splice. The major concern with belts having a mechanical splice was the damage and inaccuracies that the impact of a splice would inflict on the weighing platform, i.e., the weigh roll and load cell. However, most feeder operators have objected to the cost and down-time associated with the replacement of an endless belt and, therefore, have used spliced belts in place of endless belts.
There are a variety of spliced belts currently on the market, all of which have various deficiencies as will be discussed hereafter. The present inventors have studied spliced belts and their effect on feeder accuracy. The first thing that the inventors observed was that belts were frequently difficult to `track.` In some cases, the belts were longer on one side than the other, i.e., a conic section rather than a cylindrical section. Still other belts exhibited a lack of uniformity in the carcass ply, thus exhibiting puckering along their length or variations in straightness of the edge compared to its tracking centerline. Another important problem that had to be resolved was the drift of the v-guide located on the bottom along the centerline of travel of the belt. The v-guide of a belt with this condition rides up and out and then down and into the groove in either head or tail pulleys or both, thus shifting the distribution of belt tension. The present inventors discovered that feeders with belts exhibiting these conditions were difficult (or impossible) to calibrate and, in general, would not hold their calibration, even for a short time.
Another thing that the present inventors discovered about spliced belts was that the weight measured during a tare calibration cycle was not the weight of the belt alone but the superposition of the weight and the loadings due to moments transmitted through the belt by virtue of its stiffness and heterogeneity. This `apparent weight` of the belt has always been greater than the belt's scale weight, ranging from a high of 12 to 15% greater for old or `other source` belts to a present day low of 1% for the single ply spliced belt of the present invention. The present inventors also discovered that many things effected the apparent weight of a belt and on some belts the weight shifted wildly. The more uniform the belt, however, the less variation in apparent weight was observed.
The most desirable of belts would incorporate significant flexibility, tracking ability and homogeneity, maintain the original tensile strength (such as that exhibited in conventional endless belts), wear resistance (or wear life), and cost.
Conventional two ply belts have been used for many years and were made flexible by maintaining the thickness to around 0.31 inches. Rayon appeared to be the ideal carcass fabric due to its flexibility. But rayon presented problems to some belt manufacturers and polyester eventually became the fabric of choice. The specified belt thickness became a compromise between flexibility and wear-life; wear-life being a direct consequence of the rubber thickness between the fabric ply and the load carrying surface of the belt.
The typical construction of a two ply belt consisted of two 100 pound per inch fabric plies (approximately 1/32 of an inch thick) connected by a strip of rubber (approximately 3/64 of an inch thick) and covered on the bottom of the belt by 1/16 of an inch thick rubber and on the top by 5/32 of an inch of rubber. The two fabric plies with the 3/64 of an inch of rubber disposed therebetween behaves like a beam and adds to the stiffness of the belt.
FIG. 1 depicts a conventional spliced belt. The splice is accomplished by cutting a belt section to length from a roll of pre-vulcanized rubber belting. The belt is cut as squarely as possible and the fastener applied over the ends and stapled or riveted in place, as shown in FIG. 1. Hence, instead of the fastener laying flush to the surface of the belt, it instead forms a lump on each side of the belt, thereby interfering with the belt as it travels over the weigh rollers. Therefore, when a spliced belt moves through a gravimetric feeder, the protruding fastener plates rapidly wear down the feeder belt cleaner (i.e., scraper) contacting the top surface of the belt near the feeder discharge. The protruding fastener plate on the bottom of the belt causes the following problems: (a) it lifts the load as it passes over each of the three rolls in the weigh plane; and (b) it impacts the weigh roll in the process of moving up and over the roll. In addition, such a spliced section may cause discontinuity in belt flexibility at the junction of the two spliced halves.
Tests have demonstrated that the error which results from the lifting of the load as it passes over each of the three rolls in the weigh plane is constant only for a belt that exhibits a homogeneous distribution of tension across the belt. This will be the case only if the belt were designed and manufactured knowing the importance of a uniformly distributed belt tension. The error caused by impact to the weigh roll in the process of moving up and over the roll is a function of belt velocity. Hence, it is not constant and adds to the feeders repeatability error. The error which arises due to the discontinuity in belt flexibility derives from spikes in the apparent weight at the moment the splice aligns with the centerline of each of the three rollers of the weigh platform. The spike has been shown to be a linear function of belt tension. Spikes generated at either support roll are negative, opposite in sign and of smaller magnitude than the spike generated at the weigh roll. Various belts have been tested where the degree of canceling of the spikes varies with time and belt tension, hence, it is not constant and adds to the feeders repeatability error.
Even in a belt so reinforced, particularly one having a joint characterized as a "hinged joint", failures of the belt at the joint have been a possibility. Such failures can result from the "comb out" of the belt fastener staples through the textile fabric.
By way of further explanation, a hinged joint on a belt is a mechanical joint often used where the belt cannot be merely slipped over the ends of its pulley and must be assembled in situ. Typically, it is made by cutting the belt to length and then applying to each end a row of belt fasteners. Such belt fasteners are commercially available and well known in the art disclosed, for instance, in the U.S. Pat. No. 5,234,101 to Herold issued Aug. 10, 1993. Such a fastener comprises a U-shaped element having a bight with a belt-engaging apertured plate on either side. The joint staples are driven through the plates, piercing the belt, including the carcass ply thereinside.
Under stress there is a possibility that the staples will pull or comb out longitudinally of the belt through the fabric, much as a comb is drawn through the hair. Hence, the term "comb out". This will result in belt failure at the joint.
To deal with the "comb out" risk in the past, continuous multiple carcass plies have been used. For example, a joint in accordance with the prior art is shown in FIG. 1. Belt 1 comprises a pair of load carrying plies 2 and 3 having a layer of rubber 11 disposed therebetween. Belt ends 4 and 5 are cut off and on each is installed a row of belt fasteners 6. Belt fasteners 6 are of U-shape having a loop 7 and two plates (8, 9). Plates (8, 9) are apertured and receive staples 101 which pierce belt 1 including carcass plies 2 and 3 and are clinched on the side with the exposed points. Such belts have been durable but, because of the double plies, have not been sufficiently flexible for some purposes.
There has been a need for a highly flexible belt with a hinged belt joint which can reliably withstand continually changing tension and the stress of long-term operation without the "comb out" described above. Moreover, the present inventors have determined that one ply at approximately twice the weight (i.e., strength of one 200 lbs. per inch ply versus two 100 lbs. per inch plies) would carry the same belt tension with a significant gain in flexibility not to mention that elimination of the inner rubber ply would reduce the belts thickness by at least 3/64th of an inch. The single ply spliced belt according to the present invention was considerably more flexible than the conventional two ply spliced belts and was approximately only 0.26 inches thick. The belt design of the present invention also demonstrated a much improved signature, i.e., the incremental apparent weight as indicated by the weighing platform.
Additionally, the tensile strength of the single ply belt according to the present invention was found to be equivalent to the original two ply belts. Furthermore, since the single ply belt is made of the same rubber and thickness as the two ply belts, its wear resistance remained substantially the same. Moreover, the single ply belt of the present invention tracked as well as the two ply belts and its cost to manufacture was essentially the same.